1. Field of the Invention
The inventions disclosed and taught herein relate generally to bearings of the type that are typically used in conjunction with high speed rotating shafts, and more specifically are related to such bearing which incorporate squeeze film damper bearing support systems for attenuating vibrations within the bearings.
2. Description of the Related Art
The use of fluid film and squeeze film damping for high speed rotating shafts in turbomachinery have been used for years to add damping to rotor-bearing systems for vibration attenuation. Generally, in such systems, a thin oil film between the journal bearing housing and the bearing case provides damping by allowing the bearing housing to bounce around in the bearing case or adapter ring (hereinafter collectively referred to as “bearing case”) within the oil film. The squeeze effect on the oil produces the damping. In a fluid film bearing, a thin fluid film forms a buffer between the rotating journal surface and the stationary bearing surface, and dampens vibration from the rotor. In a squeeze film damper bearing, a thin film of fluid is squeezed by two non-rotating cylindrical surfaces. One surface is stationary while the other is positioned by a spring bar support structure and oscillates with the motion of the rotor. The squeezing of the fluid film dampens rotor vibration through the bearing support.
Damping the vibration in a turbomachine provides quiet and comfortable operation of the machine, reduced fatigue stress on the machine and its supports, and a safeguard to the damage that can be caused by unstable vibration. Vibration in a turbomachine is usually caused by a rotating mass imbalance, e.g., rotor, or by aerodynamic forces within the turbine and/or compressor. These vibrations are not static, but vary with the operating speed and operating characteristics of the turbomachine. Turbomachine vibration has a dynamic range that varies in magnitude and frequency with the operating speed of the turbomachine. An optimal bearing must is have dynamic damping characteristics tailored to the dynamic range of the vibration being applied to the bearing by the particular turbomachine.
Prior art bearings do not accommodate very well the inevitable elevation misalignments between bearings. These prior art bearings become unevenly loaded when there is misalignment with the rotor or other bearings. Misalignments can cause some bearings to bear an excessive load, while other bearings are lightly loaded. An excessive load on a fluid film bearing can reduce the thickness of the oil film to such an extent that the film is inadequate to prevent metal-to-metal contact between the bearing and the journal surface of the rotor. An inadequate oil film can cause exaggerated metal temperatures, extraordinary metal wear and premature failure of the bearing. In addition, the lightly loaded bearings may vibrate with bearing oil whirl which contributes to, rather than dampens, the vibration transmitted from the turbomachine to the bearing support. Accordingly, the inability of prior art bearings to accommodate misalignments is a serious disadvantage of these bearings.
One challenging aspect of squeeze film damper design concerns centering the bearing housing in the bearing case. To achieve acceptable damping from a squeeze film damping assembly, the non-rotatable bearing support member must be able to move within the housing. Elastomer O-rings are often used for this purpose. This is hard to achieve when, even though the O-rings are still adequate to horizontally center the movable bearing support member, the O-rings cannot support the weight of the shaft and bearing, thus permitting the movable bearing support member to rest on the bottom of the housing bore. Additionally, O-ring grooves provided in the bearing housing are offset so that the housing is high in the bearing case. When the rotor is set in the bearing, the rotor gravity load forces the bearing housing down in the bearing case. If the system is designed correctly, the bearing housing ends up being centered in the bearing case with the rotor installed.
There are numerous problems associated with using O-rings in this manner. One such problem relates to the fact that the stiffness of the O-rings is highly is nonlinear, making it difficult to choose the proper diameter and thus stiffness for bearing housing centering. Another problem is that the O-rings deteriorate over time, losing their stiffness thereby allowing the bearing housing to drop down in the bearing case, reducing the effectiveness of the damper. A further problem is that, particularly for rotors that weigh over 3,000 pounds, it is difficult to find an O-ring that will provide sufficient stiffness to counteract large rotor weight.
In order to address these problems associated with using O-rings for centering the bearing housing in the bearing case, mechanical centering devices have been developed. One traditional example of such a mechanical device employs one or more arc springs for centering. However, such systems are disadvantaged in that designing the arc spring(s) resulting in appropriate properties for the each particular bearing configuration is difficult and labor-intensive.
Squeeze film dampers in industrial turbomachinery are more typically used in higher speed machines to control the synchronous response and subsynchronous instability problems not adequately handled by conventional bearings. One recent application is reported in Leader, et al. [“The Design and Application of a Squeeze Film Damper Bearing to a Flexible Steam Turbine Rotor,” in Proceedings of the Twenty-Fourth Turbomachinery Symposium, pp. 49-58 (1995)], where an 1109 lb steam turbine rotor operating on tilting pad bearings was retrofitted with squeeze film dampers that were centered by 0-rings. This application was successful in reducing synchronous vibration amplitudes at the rotor's first critical speed by over 70 percent.
Many squeeze film damper publications exist in the literature, including Gunter, et al. [Proceedings of the Fourth Turbomachinery Symposium, pp. 127-142 (1975)], in which the fundamental damper theory is outlined. Current research includes that by San Andres regarding short squeeze film dampers with a central groove [ASME Journal of Tribology, Vol. 114 (4), pp. (1992)], who concluded that a circumferentially “ . . . grooved is damper behaves at low frequencies as a single land damper of effective length equal to the sum of the land lengths and groove width.” Development test results have also been presented by Kuzdzal, et al. [“Squeeze Film Damper Bearing Experimental vs. Analytical Results for Various Damper Configurations,” in Proceedings of the Twenty-Fifth Turbomachinery Symposium, pp. 57-70 (1996)], wherein the results compare the effectiveness of several damper centering devices. The test vehicle used was a specially modified 10 stage high pressure barrel compressor with 4.0 inch diameter tilting pad journal bearings and dummy impeller wheels. The centering devices compared included various 0-ring materials, a hanging spring arrangement, and an arc spring. The authors concluded that “ . . . an 0-ring centered damper and a mechanical spring centered damper, both with eccentricities of zero, performed well to suppress subsynchronous vibration.” A squeeze film damper tutorial containing a historical perspective, design and analysis procedures, and application examples has also been given by Zeidan, et al. [“Design and Application of Squeeze Film Dampers in Rotating Machinery,” in Proceedings of the Twenty-Fifth Turbomachinery Symposium, pp. 169-188 (1996)].
Other approaches to solving the problems surrounding the use of squeeze film dampers have been described in the patent literature. For example, U.S. Pat. No. 5,613,781 to Kuzdzal attempts to address these problems. The Kuzdzal patent discloses a damper film bearing assembly for supporting a rotatable shaft within an annular cavity formed in a housing. A fluid film damper mechanism acts between the annular outer surface of a bearing member and the outer wall of the cavity for damping radial movement of the bearing member within the cavity. A resiliently adjustable dead weight spring support system acts between the housing and the bearing member to support the dead weight of the shaft within a vertically centered position within the central bore so that the fluid film damper mechanism functions to maintain the shaft centered within the bore when vibrations occur during rotation of the shaft. The spring support system includes a bolt which engages the bearing member, which bolt is is biased away from the bearing member by a support spring (i.e., plurality of Belleville washers) in order to provide an upwardly directed force to counterbalance the weight of the shaft and the bearing and thereby resiliently urge the bearing member and the shaft upwardly into a centered position within the central bore.
While the device disclosed in the Kuzdzal patent may obviate some of the problems associated with the earlier prior art, it still suffers from a number of disadvantages of its own. For example, the Kuzdzal device is cumbersome and requires machining modifications to the bearing case. The bearing case is a difficult piece to machine as it is large, heavy, cumbersome and difficult to remove from the machine. For a retrofit, customers are reluctant to make a change that requires bearing case modifications. For the original equipment manufacture with a new machine, any changes to the bearing case would require deviation to a standard part, issuing new drawings and additional time and thus money spent on machining. Customers are often reluctant to do this.
A further disadvantage of the device described by Kuzdzal is that it would need to be assembled with the bearing case to set and check the pre-load of (i.e., the proper tightening of) the disc spring bolt and thus the proper compression of the disc springs to properly support the rotor. A additional disadvantage of the Kuzdzal device is that in order to remove the bearing from the bearing case, it is necessary to disengage the disc spring bolts. As such, the pre-load must be re-set upon re-assembly, which is time and labor intensive.
What is desired, therefore, is a centering device for squeeze film dampers which is relatively easy, and not labor intensive, to design and adapt to the configurations of particular bearings, which is not prone to losing its effectiveness over time, which provides sufficient stiffness to counteract large rotor weight, which is not cumbersome and does not require machining modifications to the bearing case, and which does not require that the pre-load be set upon assembly and re-set upon each re-assembly.
The inventions disclosed and taught herein are directed to improved centering devices for squeeze film dampers that are easy to design and adapt to configurations of particular bearings, which are not prone to losing their effectiveness over time, and which provide sufficient stiffness to counteract large rotor weight.